1. Field of the Invention
The present invention relates to an image forming apparatus of an electrophotographic type such as a copier, a printer, a facsimile machine, or a complex machine having functions of these machines, and a transfer device incorporated therein.
2. Description of the Related Art
An image forming apparatus of an electrophotographic type, especially a color image forming apparatus, forms toner images of yellow, magenta, cyan, and black sequentially on corresponding image carriers, superimposes the images on an intermediate transferring body included in a transfer device with a primary transfer unit (primary transfer operation), and then transfers the color images altogether onto a transfer medium such as a sheet of paper with a secondary transfer unit (secondary transfer operation). The image forming apparatus of an electrophotographic type are exemplified by copiers, printers, facsimile machines, and multifunction peripherals having functions of these machines. In the primary and secondary transfer operations of the image forming apparatus that incorporates the transfer device using such an intermediate transfer body, toner images formed at the previous operation need to be transferred with a high degree of fidelity.
The high-fidelity transfer can be defined as transfer of “a toner amount” and “toner image geometries” with a high degree of fidelity. First, the fidelity of the transfer of a toner amount is expressed by a transfer rate. If the rate is insufficient, the image would not satisfy an essential image quality of “image density”. The fidelity of the transfer of toner image geometries is expressed by a dot average area and dot area dispersion. If these values are insufficient, the image would not satisfy an essential image quality of “granularity (or graininess)”. If the high-fidelity transfer fails to be attained at either one of the transfer operations, i.e., the primary transfer and the secondary transfer, the final image would not come out with high image quality.
The following four documents suggest structures having a wide transfer nip to improve the toner fidelity at the primary transfer unit. Japanese Patent Application Laid-open No. 2000-292988 describes a structure incorporating a corona transfer system, in which a transfer nip is formed of an image carrier and a pressing unit arranged at the entering side of the transfer nip.
Japanese Patent Application Laid-open No. 2002-006644 discloses a structure in which a roller supporting member is provided to support a transfer belt in such a manner to be opposed to the image carrier or to support a roller at the middle of its longitudinal direction from the inside of the transfer belt.
Japanese Patent Application Laid-open No. 2002-123114 discloses a structure that includes a fixing device having a fixing roll the elastic body of which is contorted by way of a heat-resisting belt by a pressure roll provided on the inside of the heat-resisting belt to fix the toner at the same time of the transfer.
Japanese Patent Application Laid-open No. 2006-301577 relates to a high quality image technology that regulates electric discharge and reduces dust by regulating an electric field in areas upstream and downstream of the transfer nip. It teaches that, by arranging a bias application electrode of a polarity opposite to the primary transfer bias polarity near the primary transfer bias electrode, discharge is suppressed in the outlet area of the primary transfer nip to suppress polarity reversal of the toner, or more specifically reverse transfer, and to achieve faithful transfer.
Japanese Patent Application Laid-open No. 2005-181862 discloses a simultaneous double-side transfer technology, with which toner images are transferred on double sides of a recording medium at the same time in the secondary transfer operation. In particular, in the operation of transferring toner images onto the two sides of the recording medium, a toner image on the first belt, which is an intermediate transfer belt, is transferred onto the opposing second belt at the first-side image forming step. Then, another toner image is formed on the intermediate transfer belt in the second-side image forming step. The recording medium is fed at a predetermined timing, and the toner images are transferred on the two sides of the recording medium by synchronizing the leading edges of the toner images for the first and second sides while carrying the recording medium on the first and second belts. Then, the first-side toner image formed in advance on the second belt is transferred onto the recording medium by applying a negative polarity voltage to the roller at the nip outlet on the downstream side of the transfer nip and using the first nip outlet roller on the inside of the first belt as an opposing roller. On the other hand, the second-side toner image is formed on the first belt at the timing of the leading edge of the recording medium entering the transfer nip, and transferred onto the recording medium by applying a voltage of negative polarity to the first nip inlet roller and using the second belt inlet roller as an opposing roller. In other words, the first-side and second-side toner images are transferred at different positions, i.e., the transfer nip inlet and outlet areas, to simultaneously complete double-sided images.
According to the first three patent documents, the transfer nip of the transferring unit is formed with a certain curvature. Because a toner image is transferred onto the surface of a recording medium as the recording medium is being carried with a nip curvature, instability factors may be increased regarding image displacement. Furthermore, when the recording medium is a thick sheet of paper, it tends to curl up and may cause malfunction in paper feeding. When the recording medium is an electrically charged sheet such as an insulating film, it may adversely wind around the feeding roller or a belt member. Furthermore, a curled up recording medium is not a preferable condition for the user.
According to Japanese Patent Application Laid-open No. 2006-301577, a certain transfer rate needs to be maintained to perform faithful transfer without disturbing a toner image and attain a sufficient image density in the process of transferring the toner image of the intermediate transfer belt onto a recording medium in the secondary transfer operation. In the secondary transfer operation, the toner image formed on the intermediate transfer body in the primary transfer operation is transferred onto the recording medium. From the aspect of faithful transfer, a problem arises that the toner of the toner image on the intermediate transfer body dissipates toward the recording medium under the action of the secondary transfer electric field at the secondary transfer nip inlet portion, before the toner image comes into contact with the recording medium, or in other words when there still is a gap between the toner image and the recording medium. This phenomenon is called “pre-transfer”. The pre-transfer damages the original toner geometries, causing disturbances in dots and lines and generating toner dust. Another problem is a phenomenon called “separation discharge”, which occurs when the recording medium is removed from the intermediate transfer body at the secondary transfer nip outlet portion immediately after the toner image is transferred onto the recording medium. This makes the polarity of the toner image on the recording medium unstable, and may cause reverse transfer or transfer failure due to the reversely polarized toner. As a result, the original toner geometries may be damaged, leaving disturbed dots and lines and toner dust.
According to Japanese Patent Application Laid-open No. 2005-181862, the secondary transfer bias is applied for the second side of the recording medium while the toner image on the second belt for the first side is brought into contact with the recording medium. This means that the first-side toner image is more or less influenced by the secondary transfer bias. On the other hand, from an aspect of the second-side toner image transfer, the transfer rate of the second-side toner image differs, depending on the first side carrying a toner image or not. In other words, the image patterns on the front and back sides may adversely influence each other. Furthermore, each of the inlet and outlet portions of the secondary transfer nip is formed of a pair of rollers. Pressure is sufficiently applied to the roller nip portions, but the portion sandwiched by the first and second belts only does not receive sufficient contact pressure or contact tightness between the intermediate transfer body and the recording medium. This may cause disturbances in the toner images if a difference, no matter how slight it is, is created in linear velocity. In addition, when the secondary transfer electric fields are applied separately onto the front and back sides of the recording medium in the inlet or outlet portion of the secondary transfer belt nip, the transfer electric fields may interfere with each other due to the large width of the nip.